In ink-jet printing methods, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium to form characters and images. The ink-jet printing methods have been rapidly spread because of their various advantages such as easiness of full coloration, low costs, capability of using plain paper as the recording medium, non-contact with printed images and characters, etc.
Among such printing methods, in view of enhancing the weather resistance and water resistance of printed images and characters, an ink-jet printing method utilizing an ink containing a pigment as the colorant has now come to dominate. However, when using a pigment as the colorant, the printed images and characters tend to suffer from bronzing by which a reflected light therefrom is observed as a color different from an inherent color of the pigment, depending upon an angular position of the observation. For example, a reflected light from a phthalocyanine-based pigment contained in cyan inks exhibits a reddish color, resulting in considerable deterioration in quality of the resultant images. Meanwhile, details of the “bronzing” are described in “Handbook for Science of Colors”, Publishing Association of Tokyo University, p. 777.
In order to improve the bronzing, there have been proposed the ink composition containing a polyether-modified polysiloxane and a sulfone group-containing (co)polymer emulsion (JP 2003-306620A), the ink containing a polycyclic aromatic hetero-conjugated compound (JP 2004-67903A), etc. However, in these conventional techniques, there tends to occur such a risk that printed images or characters are adversely affected by changing additives and pigments added thereto.
On the other hand, there are known water dispersions and water-based inks containing colored fine particles having a core/shell structure. For example, in order to improve a durability of the water dispersions and water-based inks, there has been proposed the ink obtained by capsulating a colorant combined with a primer core/shell polymer, with a durable core/shell polymer (JP 2003-520279A); in order to improve a peel resistance of images obtained after printing, there has been proposed the ink containing colored resin particles composed of a core made of at least a colorant and a thermoplastic resin, and a shell made of a resin having a three-dimensional crosslinked structure, in a capsulated emulsion condition (JP 2002-12802A); in order to improve an ejection stability and a light resistance, there has been proposed the water dispersion containing colored fine particles as a core which contain two or more resins and a colorant (JP 2003-206430A); and in order to improve a solvent resistance, there has been proposed the dispersion of colored fine particles having a two or more-layered core/shell structure in which two or more of the cores and shell layers have a crosslinked structure (JP 2004-269558A). However, these water dispersions and water-based inks as well as the additives therefor have failed to be improved in bronzing.